The lysis of cells by directly cytotoxic T lymphocytes is a widely documented, but poorly understood, phenomenon. The proposed studies will examine several aspects of the life-history and mode of action of such T cells in a manner which may provide mechanistic insight into the recognition events that precede cytolysis. One aspect of the proposed work will focus on the fine specificity of the effector T cell's antigen receptor. Hapten-specific, H-2 restricted, cytotoxic T cells will be raised against syngeneic lymphocytes treated with trinitrobenzene sulfonic acid or by the addition of TNP-proteins to spleen cell cultures. The specificity of the killer cells induced by TNP-proteins will be assessed using target cells "coated" with either the homologous protein or with a variety of other haptenated proteins, representing both serologically cross-reacting and non cross-reacting molecules. In parallel, the specificity of classical delayed hypersensitivity responses to the same proteins will be measured. In this way, the anti-hapten specificity of two T cell-mediated events can be compared and contrasted. To further define the antigenic requirements for recognition and lysis, H-2 bearing erythrocytes, which are insusceptible to lysis by alloimmune killer T cells, will be fused with polyethylene glycol to allogeneic tumor or spleen cells. The susceptibility of the resulting cell hybrids will be tested using killer cells directed at the H-2 of the erythrocyte partner. In this way, it may prove possible to separate a cell's antigenicity from its lytic susceptibility. The influence of differentiative environment on the specificity of the effector cells generated will also be explored. In this context, the role of soluble mediators generated by mixed lymphocyte culture in the differentiation of cytotoxic memory cells will be investigated.